Re: [RFCv2] mm: page allocation for less fragmentation

Gioh Kim
2015-03-25T23:25:30Z

2015-03-26 오전 7:16에 Vlastimil Babka 이(가) 쓴 글:
> On 25.3.2015 3:39, Gioh Kim wrote:
>> My driver allocates more than 40MB pages via alloc_page() at a time and
>> maps them at virtual address. Totally it uses 300~400MB pages.
>>
>> If I run a heavy load test for a few days in 1GB memory system, I cannot allocate even order=3 pages
>> because-of the external fragmentation.
>>
>> I thought I needed a anti-fragmentation solution for my driver.
>> But there is no allocation function that considers fragmentation.
>> The compaction is not helpful because it is only for movable pages, not unmovable pages.
>>
>> This patch proposes a allocation function allocates only pages in the same pageblock.
>>
>> I tested this patch like following:
>>
>> 1. When the driver allocates about 400MB and do "cat /proc/pagetypeinfo;cat /proc/buddyinfo"
>>
>> Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
>> Node 0, zone Normal, type Unmovable 3864 728 394 216 129 47 18 9 1 0 0
>> Node 0, zone Normal, type Reclaimable 902 96 68 17 3 0 1 0 0 0 0
>> Node 0, zone Normal, type Movable 5146 663 178 91 43 16 4 0 0 0 0
>> Node 0, zone Normal, type Reserve 1 4 6 6 2 1 1 1 0 1 1
>> Node 0, zone Normal, type CMA 0 0 0 0 0 0 0 0 0 0 0
>> Node 0, zone Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0
>>
>> Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
>> Node 0, zone Normal 135 3 124 2 0 0
>> Node 0, zone Normal 9880 1489 647 332 177 64 24 10 1 1 1
>>
>> 2. The driver frees all pages and allocates pages again with alloc_pages_compact.
>
> This is not a good test setup. You shouldn't switch the allocation types during
> single system boot. You should compare results from a boot where common
> allocation is used and from a boot where your new allocation is used.

The new allocator is slower so I don't think it can replace current allocator.
I don't aim to change general allocator.
The main pupose of the new allocator is a specific allocator if system has too much fragmentation.
If some drivers consume much memory and generate fragmentation, it can use new allocator instead at the time.
I want to make a kind of compaction for drivers that allocates unmovable pages.

Therefore I tested like that.
I first generated fragmentation and called the new allocator.
I wanted to check whether the fragmentation was caused by my driver
and the pages of the driver was able to be compacted.
I thought the pages was compacted.

If I freed pages and called the commmon allocator again,
it could decrease a little fragmentation (not much as the new allocator).
But there was no pages compaction and fragmentation would increase soon.

>
>> This is a kind of compaction of the driver.
>> Following is the result of "cat /proc/pagetypeinfo;cat /proc/buddyinfo"
>>
>> Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
>> Node 0, zone Normal, type Unmovable 8 5 1 432 272 91 37 11 1 0 0
>> Node 0, zone Normal, type Reclaimable 901 96 68 17 3 0 1 0 0 0 0
>> Node 0, zone Normal, type Movable 4790 776 192 91 43 16 4 0 0 0 0
>> Node 0, zone Normal, type Reserve 1 4 6 6 2 1 1 1 0 1 1
>> Node 0, zone Normal, type CMA 0 0 0 0 0 0 0 0 0 0 0
>> Node 0, zone Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0
>>
>> Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
>> Node 0, zone Normal 135 3 124 2 0 0
>> Node 0, zone Normal 5693 877 266 544 320 108 43 12 1 1 1
>
> The number of unmovable pageblocks didn't change here. The stats for free
> unmovable pages does look better for higher orders than in the first listing
> above, but even the common allocation logic would give you that result, if you
> allocated your 400 MB using (many) order-0 allocations (since you apparently
> don't care about physically contiguous memory). That would also prefer order-0
> free pages before splitting higher orders. So this doesn't demonstrate benefits
> of the alloc_pages_compact() approach I'm afraid. The results suggest that the
> system was in a worst state when the first allocation happened, and meanwhile
> some pages were freed, creating the large numbers of order-0 unmovable free
> pages. Or maybe the system got fragmented in the first allocation because your
> driver tries to allocate the memory with high-order allocations before falling
> back to lower orders? That would probably defeat the natural anti-fragmentation
> of the buddy system.

My driver is allocating pages only with alloc_page, not alloc_pages with high order.

Yes, if I freed pages and called alloc_page again, it could decrease fragmentation at the time.
But there was no compaction and fragmentation would increase soon,
because the allocated pages was scattered all over the system.

The new allocator compacts pages. I believe it can decrease fragmentation for long time.

>
> So a proper test could be based on this:
>
>> If I run a heavy load test for a few days in 1GB memory system, I cannot
> allocate even order=3 pages
>> because-of the external fragmentation.
>
> With this patch, is the situation quantifiably better? Can you post the
> pagetype/buddyinfo for system boot where all driver allocations use the common
> allocator, and system boot with the patch? That should be comparable if the
> workload is the same for both boots.
>

OK. I'll. I can be good test.